Parting lineation (also known as current lineation or primary current lineation) is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone). The orientation of the lineation is used as a paleocurrent indicator, although the precise flow direction (i.e. upstream vs. downstream) is often indeterminable. They are also the primary indicator of the lower part of the upper flow regime bedform. Description Parting lineation is a sedimentary structure commonly found on the surface of parallel-laminated sandstones. It is aligned with the current direction, the alignment holding out in places over several square meters. The lineation is formed by flat, parallel ridges which are separated by depressions or grooves. The height of the ridges rarely exceeds several grain diameters. In profile the depressions are flat-bottomed and the ridges are rounded. Ridges and depressions are arranged en echelon, so that in downstream direction the ridges pass into depressions and vice versa. The crosswise distance between individual ridges typically varies between 0.59 and 1.25 centimeters. Length as well as spacing of the ridges augments with increasing grain size: in fine-grained sands the ridges are 3.5 to 12 centimeters long, whereas in medium-grained sands they can reach 30 centimeters in length. The long dimension of the ridges is therefore 5 to 20 times their crosswise spacing. The coarse(r) sediment fraction accumulates in the ridges, whereas dark heavy minerals and mica trails take up an intermediate position between the ridges and the grooves. Fabric Statistical studies of the spatial arrangement of the sand grains (fabric) show that in the horizontal plane the long axes of the grains form two symmetrical maxima oriented at an angle of 10 to 20 ° to the current direction. In the vertical plane these maxima are inclined at an angle of 8 to 10 ° into the current direction and the sand grains are imbricated. Genesis of the structure It is meanwhile widely accepted that parting lineation forms in the turbulent, viscous boundary layer immediately above the sediment-water interface. Responsible for the shaping of the structures are streaky vortex trains within the boundary layer. Downcurrent these streaks start to gradually lift off from the sediment surface until they eventually “burst”. Whenever this happens fluid rushes in from both sides. This cyclic process of lift-off, bursting and inrush of fluid exerts a shear